In web-building spiders, females are often too widely distributed across the landscape for males to monopolize more than one mate. Consequently, males seek females one at a time and may cohabit with females in their webs. Pre-copulatory cohabitation is most common in araneomorphs, which suggests that the first male to mate with a female will have a greater share of paternity than any subsequent mates (first sperm precedence). However, pairs of adult New Zealand sheet-web spiders (Cambridgea foliata (L. Koch, 1872); Desidae) cohabit for longer than required to achieve copulation. This is counter-intuitive as it suggests that males defend females they have already copulated with, in lieu of seeking additional mating opportunities. To investigate the costs and benefits of extended cohabitation on male and female C. foliata, we conducted surveys of webs of solitary and paired males and females. We found that solitary spiders of both sexes consistently position themselves in the center of their webs but that when in pairs, females are displaced from the webs by males and will frequently leave the web altogether. Males in pairs would respond to vibrations simulating prey, while females would not respond. This strongly suggests that extended cohabitation should be costly for females. By contrast, for males, cohabitation is a valuable foraging strategy which, combined with the advantages of mate-guarding, may compensate for any lost mating opportunities due to foregoing searching for further mates.